The present invention relates to a technical field of a control system of an engine in which a purge gas containing evaporated fuel desorbed from a canister is supplied to an intake passage.
Conventionally, arts are known in which when it is determined that evaporated fuel easily overflows from a canister during a deceleration fuel cutoff of the engine, the purge gas containing the evaporated fuel desorbed from the canister is supplied to an intake passage of the engine. For example, JP2007-198210A discloses such an art. By supplying the purge gas to the intake passage during the deceleration fuel cutoff as above, the overflow of the evaporated fuel from the canister can be suppressed. Although the evaporated fuel within the purge gas supplied to the intake passage will be discharged unburned to an exhaust passage through the engine, the unburned evaporated fuel can be purified by an exhaust emission control catalyst provided in the exhaust passage.
Further, in JP2007-198210A, a linear O2 sensor for detecting an oxygen concentration within exhaust gas for the purpose of performing a feedback control of an air-fuel ratio within a combustion chamber is provided upstream of the exhaust emission control catalyst, and an O2 sensor is provided downstream of the exhaust emission control catalyst.
Meanwhile, the O2 sensor located downstream of the exhaust emission control catalyst is normally for detecting whether a state of the air-fuel ratio of the exhaust gas is stoichiometric, rich, or lean. When the air-fuel ratio is stoichiometric or rich, an output value (output voltage) of the O2 sensor indicates a first voltage (e.g., approximately 1V), and when the air-fuel ratio is lean, the output value indicates a second voltage (e.g., approximately 0V) which is lower than the first voltage. The O2 sensor can be used for an abnormality determination in which it is determined whether the exhaust emission control catalyst is abnormal (whether it is deteriorated).
Specifically, during the deceleration fuel cutoff, oxygen is stored in the exhaust emission control catalyst, and normally during the deceleration fuel cutoff, the exhaust emission control catalyst reaches an oxygen-saturated state where oxygen cannot be stored anymore. A stored oxygen amount in the oxygen-saturated state (hereinafter, referred to as “the saturated oxygen amount”) is determined based on a volume of the exhaust emission control catalyst. Further, during the deceleration fuel cutoff, the output value of the O2 sensor indicates the second voltage.
When the operation of the engine is shifted from the deceleration fuel cutoff to a normal operation (an operation in which an injector supplies fuel to the engine and the fuel is combusted), the abnormality determination is performed. Here, a rich operation of the engine is performed. In other words, the fuel is injected by the injector so that the air-fuel ratio within the combustion chamber becomes richer than stoichiometric. An amount of excess fuel with respect to a stoichiometric amount in the injected fuel is oxidized and purified by the oxygen stored in the exhaust emission control catalyst. Therefore, as the rich operation proceeds, the oxygen stored in the exhaust emission control catalyst is consumed and the stored oxygen amount therein gradually reduces and eventually becomes zero. When the stored oxygen amount in the exhaust emission control catalyst becomes zero as above, the excess fuel cannot be purified by the exhaust emission control catalyst and the output value of the O2 sensor sharply changes from the second voltage to the first voltage. During a period from the shift of the engine operation, from the deceleration fuel cutoff to the normal operation, until the output value of the O2 sensor changes at least by a predetermined value, an integration value of the excess fuel amount is calculated. A final integration value, finally calculated when the change of the output value of the O2 sensor exceeds the predetermined value, indicates a total amount of excess fuel in the period from the shift of the engine operation to the change of the output value and corresponds to a stored oxygen amount in the exhaust emission control catalyst when the operation of the engine is shifted from the deceleration fuel cutoff to the normal operation (normally the saturated oxygen amount).
Here, if an abnormality of the exhaust emission control catalyst occurs due to, for example, deterioration, a largest storable oxygen amount in the exhaust emission control catalyst decreases below the saturated oxygen amount, and thus, the stored oxygen amount in the exhaust emission control catalyst when the operation of the engine is shifted from the deceleration fuel cutoff to the normal operation becomes lower than the saturated oxygen amount. Accordingly, the final integration value becomes lower. Therefore, the abnormality of the exhaust emission control catalyst can be determined by determining whether the final integration value indicates an excess amount below a predetermined amount.
However, by supplying the purge gas to the intake passage of the engine during the deceleration fuel cutoff (performing a purge) as in JP2007-198210A, oxygen stored in the exhaust emission control catalyst is consumed by the evaporated fuel within the purge gas during the deceleration fuel cutoff. Therefore, in the case where the purge is performed during the deceleration fuel cutoff, the stored oxygen amount in the exhaust emission control catalyst when the operation of the engine is shifted from the deceleration fuel cutoff to the normal operation is smaller than a case where the purge is not performed during the deceleration fuel cutoff. As a result, the final integration value calculated when the output value of the O2 sensor is changed at least by the predetermined value indicates an excess amount below the predetermined amount, and thus, even if the exhaust emission control catalyst is normal, it may be falsely determined as abnormal.